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French drains

Summary: Learn how to construct a french drain.

A French drain is a simple and efficient form of drainage that requires no pipe work. Water collects in a trench filled with gravel or stone, preventing ground surface water from penetrating foundations. The French drain will carry water away from the wall or property to a soak away. French drains are also often used to drain water away from retaining walls or from the outlet of a septic tank. Though easy to construct you may need a surveyor if you don't think you can get the gradient right. You may require planning permission for the drain but if not, the drain must still comply with building regulations.

Constructing a french drain

The drain must end in a soak away (see constructing a soak away).
First choose the best location for your soak away where water draining off a slope could be redirected, ideally an out-of-the-way area with sandy soil.
Check for underground cables or pipes in the area. You may need to consult your local service providers.
Plot the direction of your trench with stakes and a string line.
The drain normally begins just below the surface and runs at a gradient to the soak away. If there is no natural drainage slope, you will need to create one to direct the water. A grade of 1 in 100 is recommended i.e. a drop of 300mm (1ft) for every 30m (100ft) in length.
The trench should be a minimum of 150mm (6in) wide.
Line the trench with ground sheeting. This allows water to pass through but not soil which can block the drain over a period of time.
Fill the trench to within 10mm (1/2in) of the top with clean stone or 20mm (4/5in) gravel.
Wrap the ends of the sheeting over the top of the stone or gravel.
The top of the trench can then covered with a layer of sand and topsoil.

French drains

Constructing a french drain

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